We can rewrite the math in that condition to get a different view of what it does:
If (myDivider > (myNumber / myDivider)) Then
is the same as:
If (myDivider * myDivider > myNumber) Then
which is the same as:
If (myDivider > Math.Sqrt(myNumber)) Then
So, the condition checks if the divider is larger than the square root of the number.
The loop before that code doesn't use that fact, but you only need to check for an even divider up to the square root of the number. If the divider is larger than that, it means that the loop completed without finding an even divider, and the number is a prime.
A condition that would make more sense looking at the rest of the code would be to check for the value that the variable has after the loop:
If (myDivider = myNumber) Then
Another alternative would be to use End Sub
instead of End For
in the loop, then the condition after the loop is not needed at all.